Known in the art are systems for welding main and local pipelines (cf. V. S. Lifshits, M. D. Litvinchuk "Press Methods of Welding Main and Local Pipelines," "Nedra" Publishers, Moscow, 1970) comprising welding machines, internal and external flash removals, a device for moving the welding machines or tubes relative to each other and a mobile power source. Mobile diesel-electric units are used as a power supply in these systems. Thus, a 500 -kw diesel-electric unit is used for welding tubes 1020 mm in diameter. In the field the diesel-electric unit is run at a maximum speed with an output voltage of the synchronous generator of 400 V and a frequency of 50 Hz during the whole working shift regardless of the load. At a rated load of 500 kW the diesel-electric unit is in operation not more than 2% of the total working time, while the remaining period is characterized by loads below the rated one in the intervals between the welding operations, when only the auxiliary mechanisms are in operation. The total input power of these mechanisms is one tenth of the rated power. This cuts down the life of the diesel engine of the unit. Furthermore, a high installed rated power of the units hinders their transportation and supply with fuel under field conditions.
In addition, in the process of welding it is desirable to change the frequency of the current fed to the welding transformer. However, the presence of a single power source for the welding transformer, a-c motors and auxiliary mechanisms makes this change impossible. This reduces the thermal efficiency of the welding process (longer welding time and higher consumption of the tube material) and rises its cost.